System and method for direct storage access in a content-centric network

ABSTRACT

One embodiment of the present invention provides a system for caching content data to a storage device attached to a node in a content-centric network (CCN). During operation, the system receives a content packet; forwards the content packet to an incoming port of an interest in the content packet; caches a copy of the content packet in the attached storage device, assembles a query; which includes at least a network header and an address associated with storage blocks at which the cached copy of the content packet is located, corresponding to the content packet; and stores the query in a cache table, thereby facilitating a subsequent direct access to the storage blocks using the assembled query to retrieve the copy of the content packet.

RELATED APPLICATIONS

The subject matter of this application is related to the subject matterin the following applications:

-   -   U.S. patent application Ser. No. 14/065,961 (Attorney Docket No.        PARC-20130997US01), entitled “SYSTEM AND METHOD FOR HASH-BASED        FORWARDING OF PACKETS WITH HIERARCHICALLY STRUCTURED        VARIABLE-LENGTH IDENTIFIERS,” by inventors Marc E. Mosko and        Michael F. Plass, filed 29 Oct. 2013;    -   U.S. patent application Ser. No. 14/067,857 (Attorney Docket No.        PARC-20130874US01), entitled “SYSTEM AND METHOD FOR MINIMUM PATH        MTU DISCOVERY IN CONTENT CENTRIC NETWORKS,” by inventor Marc E.        Mosko, filed 30 Oct. 2013; and    -   U.S. patent application Ser. No. 14/069,286 (Attorney Docket No.        PARC-20130998US02), entitled “HASH-BASED FORWARDING OF PACKETS        WITH HIERARCHICALLY STRUCTURED VARIABLE-LENGTH IDENTIFIERS OVER        ETHERNET,” by inventors Marc E. Mosko, Ramesh C. Ayyagari, and        Subbiah Kandasamy, filed 31 Oct. 2013;        the disclosures of which herein are incorporated by reference in        their entirety.

BACKGROUND

1. Field

The present disclosure relates generally to facilitating storage accesswithin a data network. More specifically, the present disclosure relatesto a system and method for facilitating direct storage access in acontent-centric network (CCN).

2. Related Art

The proliferation of the Internet and e-commerce continues to fuelrevolutionary changes in the network industry. Today, a significantnumber of information exchanges, from online movie viewing to daily newsdelivery, retail sales, and instant messaging, are conducted online. Anincreasing number of Internet applications are also becoming mobile.However, the current Internet operates on a largely location-basedaddressing scheme. The two most ubiquitous protocols, the InternetProtocol (IP) and Ethernet protocol, are both based on location-basedaddresses. That is, a consumer of content can only receive the contentby explicitly requesting the content from an address (e.g., IP addressor Ethernet media access control (MAC) address) closely associated witha physical object or location. This restrictive addressing scheme isbecoming progressively more inadequate for meeting the ever-changingnetwork demands.

Recently, content-centric network (CCN) architectures have been proposedin the industry. CCN brings a new approach to content transport. Insteadof having network traffic viewed at the application level as end-to-endconversations over which content travels, content is requested orreturned based on its unique name, and the network is responsible forrouting content from the provider to the consumer. Note that contentincludes data that can be transported in the communication system,including any form of data such as text, images, video, and/or audio. Aconsumer and a provider can be a person at a computer or an automatedprocess inside or outside the CCN. A piece of content can refer to theentire content or a respective portion of the content. For example, anewspaper article might be represented by multiple pieces of contentembodied as data packets. A piece of content can also be associated withmetadata describing or augmenting the piece of content with informationsuch as authentication data, creation date, content owner, etc.

In CCN, content objects and interests are identified by their names,which is typically a hierarchically structured variable-lengthidentifier (HSVLI). When an interest in a piece of content is receivedat a CCN node, a local content cache is checked to see if the contentbeing requested exists. In addition, the CCN node may selectively cachepopular content objects to increase the network response rate. In orderto cache large amount of content, a CCN router or switch may couple tostorage devices, which may be organized as a direct-attached storage(DAS) system, a network-attached storage (NAS) system, and or a storagearea network (SAN) system.

SUMMARY

One embodiment of the present invention provides a system for cachingcontent data to a storage device attached to a node in a content-centricnetwork (CCN). During operation, the system receives a content packet;forwards the content packet to an incoming port of an interest in thecontent packet; caches a copy of the content packet in the attachedstorage device, assembles a query; which includes at least a networkheader and an address associated with storage blocks at which the cachedcopy of the content packet is located, corresponding to the contentpacket; and stores the query in a cache table, thereby facilitating asubsequent direct access to the storage blocks using the assembled queryto retrieve the copy of the content packet.

In a variation on this embodiment, the attached storage device includesat least one of: an advanced technology attachment (ATA) over Ethernetstorage array, a Fibre Channel over Ethernet (FCoE) storage array, andan Internet Small Computer System Interface (iSCSI) storage array.

In a further variation, the network header includes an Ethernet header.

In a variation on this embodiment, the content packet includes aplurality of fragments. The system caches a copy of the content packetby storing each fragment into a separate set of contiguous storageblocks, and assemble a separate query for each fragment. An entry in thecache table corresponding to the content packet includes a plurality ofqueries corresponding to the plurality of fragments.

In a variation on this embodiment, the system subsequently receives aninterest in the content packet; searches the cache table to obtain thequery; sends the query to the attached storage device; receives, fromthe attached storage device, a data packet in response to the query;removes a network header from the received data packet; and forwards thedata packet.

In a further variation, searching the cache table involves performing alongest-prefix-match lookup based on the received interest.

In a further variation, the system further constructs a networkrepresentation of the query based on elements stored in the cache table

In a variation on this embodiment, the address associated with thestorage blocks includes a logical block addressing (LBA) address.

In a variation on this embodiment, the node includes multiple line cardseach capable of receiving content and interest packets. The systemfurther populates the cache table to all of the multiple line cards.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an exemplary architecture of a network, in accordancewith an embodiment of the present invention.

FIG. 2 presents a diagram illustrating an exemplary content-centricnetwork (CCN)-enabled switch with attached storage, in accordance withan embodiment of the present invention.

FIG. 3 presents a diagram illustrating an exemplary line card in aCCN-enabled switch, in accordance with an embodiment of the presentinvention.

FIG. 4 presents a diagram illustrating an exemplary cache table storedin the content store, in accordance with an embodiment of the presentinvention.

FIG. 5 presents an exemplary Ethernet frame stored in the cache tablefor direct storage access, in accordance with an embodiment of thepresent invention.

FIG. 6 presents a diagram illustrating an exemplary Content Objectfragment retrieved from the attached storage, in accordance with anembodiment of the present invention.

FIG. 7 presents a flowchart illustrating the process of caching aContent Object in an attached storage device, in accordance with anembodiment of the present invention.

FIG. 8 presents a flowchart illustrating the process of retrieving acached Content Object from an attached storage device, in accordancewith an embodiment of the present invention.

FIG. 9 illustrates an exemplary system for direct access to attachedstorage, in accordance with an embodiment.

In the figures, like reference numerals refer to the same figureelements.

DETAILED DESCRIPTION Overview

Embodiments of the present invention provide a system and method forquick, efficient access to a storage that is attached to a CCN-enabledswitch or router. During operation, when a CCN-enabled switch or routerreceives a content object or fragments of the content object, the switchdetermines that the content object should be cached, and sends a copy ofthe content object (or its fragments) to a content store manager. Thecontent store manager reassembles the fragments (when needed), andwrites the copy of the content object into the attached storage. Morespecifically, before writing into the attached storage, the contentstore manager may also fragment the content object to suitable size fortransportation. Each transported fragment is written into a contiguousset of blocks in the attached storage, and the content store manageradds transport and fragment headers to each contiguous set of blocks. Inaddition, the content store manager generates and populates a cachetable indexed by the name prefix of the content object. An entry in thecache table specifies locations of the sets of storage blocks at whichfragments of the content object are located. In some embodiments, acache entry includes a set of pre-assembled data communication frameswith each frame dedicated to a fragment. A pre-assembled frame mayinclude appropriate transport headers and a pointer to the storageblocks, and can be used to retrieve the corresponding fragment from theattached storage.

In general, CCN uses two types of messages: Interests and ContentObjects. An Interest carries the hierarchically structuredvariable-length identifier (HSVLI), also called the “name,” of a ContentObject and serves as a request for that object. If a network element(e.g., router) receives multiple interests for the same name, it mayaggregate those interests. A network element along the path of theInterest with a matching Content Object may cache and return thatobject, satisfying the Interest. The Content Object follows the reversepath of the Interest to the origin(s) of the Interest. A Content Objectcontains, among other information, the same HSVLI, the object's payload,and cryptographic information used to bind the HSVLI to the payload.

The terms used in the present disclosure are generally defined asfollows (but their interpretation is not limited to such):

-   -   “HSVLI:” Hierarchically structured variable-length identifier,        also called a Name. It is an ordered list of Name Components,        which may be variable length octet strings. In human-readable        form, it can be represented in a format such as ccnx:/path/part.        There is not a host or query string. As mentioned above, HSVLIs        refer to content, and it is desirable that they be able to        represent organizational structures for content and be at least        partially meaningful to humans. An individual component of an        HSVLI may have an arbitrary length. Furthermore, HSVLIs can have        explicitly delimited components, can include any sequence of        bytes, and are not limited to human-readable characters. A        longest-prefix-match lookup is important in forwarding packets        with HSVLIs. For example, an HSVLI indicating an interest in        “/parc/home/bob” will match both “/parc/home/bob/test.txt” and        “/parc/home/bob/bar.txt.” The longest match, in terms of the        number of name components, is considered the best because it is        the most specific.    -   “Interest:” A request for a Content Object. The Interest        specifies an HSVLI name prefix and other optional selectors that        can be used to choose among multiple objects with the same name        prefix. Any Content Object whose name matches the Interest name        prefix and selectors satisfies the Interest.    -   “Content Object:” A data object sent in response to an Interest.        It has an HSVLI name and a Contents payload that are bound        together via a cryptographic signature. Optionally, all Content        Objects have an implicit terminal name component made up of the        SHA-256 digest of the Content Object. In one embodiment, the        implicit digest is not transferred on the wire, but is computed        at each hop, if needed.

As mentioned before, an HSVLI indicates a piece of content, ishierarchically structured, and includes contiguous components orderedfrom a most general level to a most specific level. The length of arespective HSVLI is not fixed. In content-centric networks, unlike aconventional IP network, a packet may be identified by an HSVLI. Forexample, “abcd/bob/papers/ccn/news” could be the name of the content andidentifies the corresponding packet(s), i.e., the “news” article fromthe “ccn” collection of papers for a user named “Bob” at theorganization named “ABCD.” To request a piece of content, a nodeexpresses (e.g., broadcasts) an interest in that content by thecontent's name. An interest in a piece of content can be a query for thecontent according to the content's name or identifier. The content, ifavailable in the network, is routed back to it from any node that storesthe content. The routing infrastructure intelligently propagates theinterest to the prospective nodes that are likely to have theinformation and then carries available content back along the path whichthe interest traversed.

FIG. 1 illustrates an exemplary architecture of a network, in accordancewith an embodiment of the present invention. In this example, a network180 comprises nodes 100-145. Each node in the network is coupled to oneor more other nodes. Network connection 185 is an example of such aconnection. The network connection is shown as a solid line, but eachline could also represent sub-networks or super-networks, which cancouple one node to another node. Network 180 can be content-centric, alocal network, a super-network, or a sub-network. Each of these networkscan be interconnected so that a node in one network can reach a node inother networks. The network connection can be broadband, wireless,telephonic, satellite, or any type of network connection. A node can bea computer system, an end-point representing users, and/or a device thatcan generate interest or originate content.

In accordance with an embodiment of the present invention, a consumercan generate an Interest in a piece of content and then send thatInterest to a node in network 180. The piece of content can be stored ata node in network 180 by a publisher or content provider, who can belocated inside or outside the network. For example, in FIG. 1, theInterest in a piece of content originates at node 105. If the content isnot available at the node, the Interest flows to one or more nodescoupled to the first node. For example, in FIG. 1, the Interest flows(interest flow 150) to node 115, which does not have the contentavailable. Next, the Interest flows (interest flow 155) from node 115 tonode 125, which again does not have the content. The Interest then flows(interest flow 160) to node 130, which does have the content available.The flow of the content then retraces its path in reverse (content flows165, 170, and 175) until it reaches node 105, where the content isdelivered. Other processes such as authentication can be involved in theflow of content.

In network 180, any number of intermediate nodes (nodes 100-145) in thepath between a content holder (node 130) and the Interest generationnode (node 105) can participate in caching local copies of the contentas it travels across the network. Caching reduces the network load for asecond subscriber located in proximity to other subscribers byimplicitly sharing access to the locally cached content.

CCN-Enabled Switch with Attached Storage

As described previously, in CCN, it is desirable to have intermediatenodes caching local copies of the content. This requires theintermediate nodes to have a large storage capacity because the amountof content flow through the network can be huge. For example, a storagedevice with a large-capacity, such as a capacity of many terra-bytes,would be needed to maintain a video cache. Such a large-capacity storageis usually implemented in the form of attached storage, such asnetwork-attached storage (NAS).

FIG. 2 presents a diagram illustrating an exemplary content-centricnetwork (CCN)-enabled switch with attached storage, in accordance withan embodiment of the present invention. In FIG. 2, CCN-enabled switch200 includes a number of line cards, such as line cards 202, 204, and206; and a supervisor card 208. In FIG. 2, CCN-enabled switch 200 iscoupled to an attached storage array 210. Attached storage array 210 caninclude various types of disk arrays, which can be a network attachedstorage (NAS) array, a storage area network (SAN) array, or avirtualized storage array. In some embodiments, each line card (such asline card 202, 204, or 206) in CCN-enabled switch 200 is coupled toattached storage array 210 via a high-speed switch 212. In someembodiments, supervisor card 208 is coupled to, and has direct access tostorage array 210, and the line cards are configured to access storagearray 210 via supervisor card 208.

To facilitate efficient content retrieval, attached storage array 210can operate at high speed. In some embodiments, attached storage array210 includes a high-speed interface, such as a 10 Gigabit Ethernet (10GE) interface, for interfacing with CCN-enabled switch 200. In addition,depending on the types of storage arrays, such as advanced technologyattachment (ATA) storage arrays or fibre channel (FC) storage arrays,the communication between storage array 210 and CCN-enabled switch 200can use different network protocols, such as ATA over Ethernet (AoE),Fibre Channel (FC), or Fibre Channel over Ethernet (FCoE). In addition,other IP-based protocols, such as iSCSI (Internet Small Computer SystemInterface), can also be used to enable communication between an IP-basedattached storage with CCN-enabled switch 200.

FIG. 3 presents a diagram illustrating an exemplary line card in aCCN-enabled switch, in accordance with an embodiment of the presentinvention. In FIG. 3, line card 300 includes a number of media ports,such as media ports 302, 304, and 306; a network-processing unit (NPU)308, a content store (CS) manager 310, and a content store 312.

During operation, media ports 302-306 are responsible for interfacingwith external devices, such as other hosts or an attached storage array.For example, media ports 302 and 306 are coupled to other hosts and areresponsible for sending/receiving Interest or Content Object to/from theother hosts, and media port 304 is coupled to an attached storage arrayand is responsible for caching/retrieving Content Objects to/from theattached storage array.

NPU 308 has a limited computation capability, and is responsible forprocessing the incoming/outgoing packets at line speed, i.e., at thesame speed the packets being received. In some embodiments, NPU 308 isresponsible for examining a receiving Interest packet and performsprefix match lookup in various data structures, such as content store312, a pending interest table (PIT), and a forwarding information base(FIB). Note that the PIT and the FIB are not shown in FIG. 3. In someembodiments, the PIT, the FIB, and content store 312 can be implementedusing random-access memories (RAMs), which can be a dynamic RAM (DRAM)or a static RAM (SRAM).

Content Store (CS) 312 is similar to the buffer memory used in an IProuter. In conventional CCNs, a CS may temporarily buffers ContentObjects that pass through this switch. However, such an approach meansthat the size of the CS will determine the amount of content data thatcan be cached by the switch. In contrast, in some embodiments of thepresent invention, instead of keeping the content data, CS 312 onlymaintains a cache table that can be used for retrieval of ContentObjects stored in the attached storage array. Details about the cachetable will be described later.

The PIT keeps track of Interests forwarded upstream toward contentsource(s) so that returned Content Object can be sent downstream to itsrequester(s). In CCN, only Interest packets are routed. The returningContent Object follows the trail of the Interest packet back to thecontent requester. A PIT entry for an Interest specifies the name of theInterest and one or multiple incoming and outgoing ports for thatInterest. Multiple ports listed in the PIT entry indicate that the samecontent has been requested by multiple downstream users, and multipleoutgoing ports indicate that the same Interest is forwarded alongmultiple paths.

The FIB is used to forward Interest packets toward potential source orsources of matching Content Objects. Typically, a routing protocol isused to populate the FIB among all nodes in the network. The FIB entriesare often indexed by the name prefixes.

When an Interest packet arrives on a media port 302 from a remote host,NPU 308 performs one or more longest-match lookups based on the Interestname, or the HSVLI. In some embodiments, hash-based forwarding is used,where each node uses the same hash function to encode name prefixes andperforms longest-match lookup using the fixed-length hash. Detaileddescriptions of the hash forwarding can be found in U.S. patentapplication Ser. No. 14/065,961 (Attorney Docket No. PARC-20130997US01),entitled “SYSTEM AND METHOD FOR HASH-BASED FORWARDING OF PACKETS WITHHIERARCHICALLY STRUCTURED VARIABLE-LENGTH IDENTIFIERS,” by inventorsMarc E. Mosko and Michael F. Plass, filed 29 Oct. 2013, the disclosureof which herein is incorporated by reference in its entirety.

In some embodiments, the index structure used for the name lookup isordered in a way such that a CS match will be preferred over a PITmatch, which will be preferred over an FIB match. Hence, if there isalready a matching entry found in CS 312, the Content Object will beretrieved and sent out the port the Interest arrived on (such as mediaport 302) and the Interest will be discarded. Otherwise, NPU 308 checksthe PIT to see if a matching entry can be found. If so, the Interest'sarrival port will be added to the PIT entry's requesting port list andthe Interest will be discarded. Otherwise, the FIB will be checked. Notethat for system relying on hash forwarding, the FIB entries may beindexed based on the forwarding hashes, and checking the FIB involvesfinding a longest matching prefix in the FIB. If there is a match in theFIB, then the Interest needs to be forwarded to the ports listed in theFIB entry. For example, the FIB may list media port 306 as the outgoingport for the received Interest, and consequently, NPU 308 forwards theInterest to a corresponding host via media port 306.

When the requested Content Object is returned, it arrives at line card300 on media port 306. For a Content Object of large size, fragments ofthe Content Object will be received at media port 306. Subsequently, NPU308 processes the Content Object (or fragments of the Content Object) bymatching its name, such as the HSVLI or its hash, in the PIT. Note thatNPU 308 may drop the Content Object if no matching entry is found in thePIT. In addition, NPU 308 may also determine whether the Content Objectarrives from a port over which a corresponding Interest (based on thematching entry in the PIT) was previously forwarded, or over which thecorresponding Interest could have been forwarded. If this condition isnot met, NPU 308 drops the Content Object. The ingress port in thematching PIT entry is used to forward the Content Object (or itsfragments), thus ensuring that the Content Object is reverse-pathforwarded back to the host sending the Interest. In addition toforwarding the Content Object, NPU 308 determines whether to cache theContent Object. In some embodiments, NPU 308 caches every Content Objectpassing through. In some embodiments, NPU 308 may determine whether tocache a Content Object based on its popularity or importance, and cachesContent Objects having a popularity/importance level exceeding a certainthreshold (such as the number of requesters exceeding a thresholdnumber).

Once NPU 308 determines to cache a Content Object, it sends a copy ofthe Content Object to CS manager 310. In some embodiments, NPU 308 maymulticast the Content Object to the outgoing port (media port 306) andCS manager 310. CS manager 310 can be a process running on a centralprocessing unit (CPU), which can be located on line card 300 or on thesupervisor card. In some embodiments, CS manger 310 may be implementedusing hardware. CS manager 310 processes the Content Object as neededand sends the processed Content Object to the attached storage array viamedia port 304.

Depending on the size of the Content Object and the type of storagearray, CS manager 310 may perform different operations on the ContentObject. For example, the received Content Object may be fragmented byone or more of the previous links in order to meet the maximumtransmission unit (MTU) requirements of those links. Detaileddescription of the fragmentation of Content Objects in CCNs can be foundin U.S. patent application Ser. No. 14/067,857 (Attorney Docket No.PARC-20130874US01), entitled “SYSTEM AND METHOD FOR MINIMUM PATH MTUDISCOVERY IN CONTENT CENTRIC NETWORKS,” by inventor Marc E. Mosko, filed30 Oct. 2013, the disclosure of which herein is incorporated byreference in its entirety. In some embodiments, upon receiving thefragments of a Content Object, CS manager 310 may reassemble theoriginal Content Object based on the fragment headers.

In addition to reassembling a fragmented Content Object, in someembodiments, CS manager 310 also performs packet fragmentation. Thistime, the purpose of the fragmentation is to ensure that each fragmentis in a suitable size for subsequent network transportation, moreparticularly, satisfying the MTU requirement imposed by the networktransport protocol in possible subsequent links. Depending on theprotocol used and the underlying physical transmission medium, differentnetwork links may impose different MTUs. For example, IPv6 requires thatall of its data links support a minimum MTU of 1280 bytes. Ethernet datalinks typically have an MTU of 1500 bytes.

If the transport protocol for a network link requires an MTU of 1280bytes, the Content Object may need to be divided to a plurality offragments of 1200 bytes each. CS manager 310 then writes each fragment,along with the transport and fragment headers, to a set of contiguousblocks in the attached storage array. For example, if the attachedstorage array includes ATA drives, three contiguous ATA blocks areneeded to cache a fragment that is transported over the 1280-byte MTUlink. Note that the blocks include the fragment of the Content Objectand the transport and fragment headers. The contiguous set of blocksshould also fit within the local attachment size. If the communicationprotocol on the link between line card 300 and the attached storagearray is AoE or FCoE, meaning the contiguous set of blocks should fitwithin the Ethernet frame size of 1500 bytes or the jumbo Ethernet framesize of 9000 bytes.

In addition to caching the fragments of the Content Object into theattached storage array, CS manager 310 is also responsible forgenerating and populating (among all line cards within the switch node)a cache table. The cache table can be stored in CS 312, and are indexedby Interest or CCN names. A respective entry in the cache table matchesan Interest in a corresponding Content Object. More specifically, theentry specifies the sets of storage blocks at which fragments of theContent Object are cached. In some embodiments, CS manager 310 canpre-assemble a set of queries that can later be used for retrieval ofthe sets of storage blocks. By pre-assembling the set of queries, CSmanager 310 eliminates the need of query computation at the time whenthe Content Object is requested, thus making it possible for NPU 308(which often has a much limited computation capability than that of aCPU, and in general does not include a file system) to process asubsequently arrived Interest and retrieve the Content Object at linespeed.

In some embodiments, instead of the pre-assembled queries, CS 312 storesa compressed table that includes an index to the storage array's networkaddress, a flag to indicate the type of encapsulation to construct, andthe disk addresses to retrieve. When subsequent Interest in the ContentObject arrives, NPU 308 constructs the appropriate network frame orframes. This approach saves storage space in CS 312 but requiresadditional NPU processing cycles.

In some embodiments, the network protocol used by the link between linecard 300 and the attached storage array is AoE or FCoE, meaning that thepre-assembled queries are pre-assembled Ethernet frames that includesEthernet headers as well as headers specific to the type of storage,such as ATA headers or FC headers. FIG. 4 presents a diagramillustrating an exemplary cache table stored in the content store, inaccordance with an embodiment of the present invention. In FIG. 4, cachetable 400 includes a plurality of name-indexed entries. Each entry, suchas an entry 402, corresponds to a Content Object, and includes a name(such as /ABC/DEF.COM) and a set of Ethernet frames. The name can be thename of the Content Object, or the name of an Interest to which theContent Object is returned. For systems using hash-forwarding, the namecan be the fixed-length hash function of the HSVLI of the Content Objector the corresponding Interest. Each Ethernet frame in the entry (such asFRAME_(—)1 to FRAME_N) corresponds to a fragment of the Content Object.For example, if a Content Object is fragmented to three fragments whenit is written into the attached storage array, the entry correspondingto the Content Object in cache table 400 will include three Ethernetframes, with each frame corresponding to a fragment.

In some embodiments, a pre-assembled Ethernet frame in the cache tableincludes Ethernet wrappers, fragment headers, and a pointer to thestorage blocks that stores the fragment. FIG. 5 presents an exemplaryEthernet frame stored in the cache table for direct storage access, inaccordance with an embodiment of the present invention. In FIG. 5,Ethernet frame 500 includes a logical block addressing (LBA) address502, a fragment header 504, and an Ethernet header 506.

LBA address 502 specifies the address of the set of contiguous storageblocks which stores the content fragment. Note that depending on thetype of storage, certain specific command format may be used. Forexample, ATA read command may be used to encompass LBA address 502.Those specific formats or storage headers are not shown in FIG. 5.Fragment header 504 includes information associated with thefragmentation, such as the fragment stream ID identifying the ContentObject, total number of fragments in the stream, and the fragment serialnumber identifying each fragment. Ethernet header 506 includesinformation needed for sending the frame to the correct destination,such as the media access control (MAC) address of the destination (whichcan be a physical attached storage device or a virtual storage).

When an Interest requesting the cached Content Object is received at anode, the cache table stored in the CS is checked to determine whether amatching entry can be found. In some embodiments, a longest-prefix-matchlookup is performed. Once a matching entry is found, the node sendsevery Ethernet frame included in the matching entry to the attachedstorage device to retrieve all fragments of the Content Object. Notethat, because the Ethernet frames have been pre-assembled, there is noneed to perform any computation associated with assembly ofcommunication frames suitable for retrieving the fragments. Hence,instead of invoking the powerful switch CPU, the NPU, which has limitedcomputation capacity, is sufficient in processing the received Interestand retrieving the cached Content Object. In addition, the simplicity ofsending previously generated communication frames (such as Ethernetframes) also means that the cached Content Object can be retrieved at amuch faster rate compared to a scenario where a frame suitable forcommunication with the attached storage is generated only when cacheretrieval is needed.

Once those pre-assembled Ethernet frames are sent to the attachedstorage device, the control and management module of the storage deviceanalyzes the received frames to obtain the LBA addresses included in theframes. Using the obtained LBA addresses, the control and managementmodule can access the corresponding storage blocks to obtain the cachedcontent. Note that each Ethernet frame can be used to retrieve acorresponding content fragment, which can occupy multiple contiguousstorage blocks as specified by the LBA address. For ATA over Ethernet, aretrieved content packet can include the ATA over Ethernet header. FIG.6 presents a diagram illustrating an exemplary Content Object fragmentretrieved from the attached storage, in accordance with an embodiment ofthe present invention.

In FIG. 6, a retrieved Content Object fragment 600 includes a contentdata field 602, a fragment header 604, and an ATA over Ethernet header606. Content data field 602 includes the content fragment. Fragmentheader 604 is similar to fragment header 504, and includes informationassociated with the fragmentation. ATA over Ethernet header 606 includesinformation associated with the Ethernet link, such as the Ethernetsource and destination addresses, and ATA-specific fields, such assector count and LBA addresses.

FIG. 7 presents a flowchart illustrating the process of caching aContent Object to an attached storage device, in accordance with anembodiment of the present invention. During operation, an intermediatenode (such as a switch or a router) coupled to an attached storage in aCCN receives a Content Object or fragments of the Content Object(operation 702). Note that the intermediate node can be a CCN-enabledswitch that maintains the three major CCN data structures, the contentstore (CS), the pending Interest table (PIT), and the forwardinginformation base (FIB). In addition, the Content Object may have beenfragmented by its originating node to ensure that no furtherfragmentation is needed along the path of the Content Object.

The intermediate node then identifies, based on the name or name prefixof the Content Object, a matching Interest in the PIT (operation 704),and forwards the Content Object to the one or more requesting ports(which are the incoming ports of the matching pending Interest) listedin the PIT entry (operation 706). In some embodiments, the PIT lookupand the Content Object forwarding are performed by a simplenetwork-processing unit (NPU) at the line speed. The intermediate nodealso determines whether to cache the received Content Object (operation708). In some embodiments, the NPU makes such decision based on thepopularity or the importance level of the Content Object. In response tothe NPU determining to cache the Content Object, the NPU sends a copy ofthe received Content Object to a content store (CS) manager (operation710). In some embodiments, the Content Object is multi-casted to therequesting ports (as listed in the PIT) and the CS manager. In someembodiments, the CS manager can be a process running on the switch CPUor a standalone module. Upon receiving the Content Object or fragmentsof the Content Object, the CS manager may optionally fragment (orre-fragment if it receives fragments initially) the Content Object basedon the MTU of the link between the intermediate node and the attachedstorage. For each fragment, the CS manager encapsulates the content datawith appropriate transport and fragment headers (operation 714), andsends the encapsulated content data frame to the attached storage device(operation 716). Note that, to do so, the CS manager may need todetermine the number of contiguous storage blocks needed for caching thecontent data frame. The appropriate headers may include the fragmentheaders (if the Content Object is fragmented) and the transport headers.Subsequently, the CS manager generates a communication frame that canlater be used to retrieve the cached content fragment from the attachedstorage (operation 718). When all fragments of a Content Object havebeen cached, the CS manager creates an entry in a cache table (operation720). The entry is indexed by the corresponding Interest (as indicatedby the PIT entry). Alternatively, entries in the cache table can also beindexed by names of the cached Content Objects. Note that in a systemusing hash-based forwarding, the entries may be indexed by thesimilarity hash of the Interest. Note that the similarity hash iscomputed to uniquely identify a piece of content, and can be a hash ofthe name and one or more fields in the content packet. If theintermediate node includes multiple line cards capable of receivingInterests, the CS manager (which can be located at the supervisor card)will populate the cache table to all line cards, thus ensuring that thesame cache table is checked for Interests received on any line card.

FIG. 8 presents a flowchart illustrating the process of retrieving acached Content Object from an attached storage device, in accordancewith an embodiment of the present invention. During operation, aCCN-enabled switch receives an Interest (operation 802), and determineswhether a matching entry exists in the PIT (operation 804). In someembodiments, the Interest may arrive on a media port located on a linecard of the switch, and the operation of checking the PIT may beperformed by a NPU included in the same line card. In furtherembodiments, checking the PIT may include perform a longest-match lookupbased on a HSVLI associated with the Interest. If so, the NPU adds theport on which the Interest is received to the incoming port listed inthe corresponding PIT entry (operation 806). If not, the NPU furtherchecks the Content Store to determining whether a matching entry can befound in a cache table maintained at the Content Store (operation 808).In some embodiments, the cache table is stored in a fast memory, anddifferent entries in the cache table represent different Content Objectsstored in a storage attached to the CCN-enabled switch. The entries areindexed by names of Interests (HSVLIs or their hashes) that correspondto the Content Objects. A respect entry in the cache table includes anumber of communication frames, each capable of retrieving a contentfragment from the attached storage. In some embodiments, the attachedstorage is an ATA over Ethernet (AoE) storage or a Fibre Channel overEthernet (FCoE) storage, and the communication frames are Ethernetframes.

If a matching entry is found in the cache table, the NPU sends allcommunication frames (which can be Ethernet frames) included in thematching entry to the attached storage to retrieve corresponding contentfragments (operation 810). Note that, in some embodiments, the totalnumber of frames included in each entry corresponds to the total numberof fragments included in the Content Object. Once a content fragment isretrieved from the attached storage, the NPU removes the encapsulatedtransport headers, such as AoE or FCoE headers (operation 812), andsends the content data (optionally including the fragment header) to theincoming port of the Interest (operation 814). In some embodiments, theNPU may identify frames whose encapsulated transport headers should beremoved based on whether the frame has a destination Ethernet address asthe NPU and a source Ethernet as from a known storage system. Note thatthe payload of the received AoE or FCoE packets are properly formed CCNContent Object or Content Object fragment, no additional processing isnecessary. If no matching entry is found in the cache table, the NPUwill forward the Interest using information obtained from the FIB(operation 816).

Note that, in embodiments of the present invention, the queries orcommunication frames that can directly access storage blocks located onthe attached storage are generated before the actual querying happens.Also note that in order to directly read from those storage blocks, thequeries need to abide to certain appropriate format depending on thetype of the attached storage. For AoE or FCoE storage, Ethernet headersare needed to encapsulate the ATA or Fibre Channel read command. Such anencapsulation can take time and delay the response to an incomingInterest. Hence, by pre-assembling (using a more powerful CPU)appropriate queries and caching those pre-assembled queries in a fastmemory, embodiments of the present invention allow fast contentretrieval. More specifically, a simple NPU, which has limitedcomputation power and does not have a file system, can directly accessthe attached storage for retrieval of cached Content Object. Note thatalthough computationally limited, the NPU is capable of performingsimple operations of table lookup and frame forwarding at a much higherspeed than the frame-assembling speed of a powerful CPU, thus ensuring afast Interest response rate, which is desirable for CCNs.

FIG. 9 illustrates an exemplary system for direct access to attachedstorage, in accordance with an embodiment. A system 900 for directaccess to attached storage comprises a processor 910, a memory 920, anda storage 930. Storage 930 typically stores instructions which can beloaded into memory 920 and executed by processor 910 to perform themethods mentioned above. In one embodiment, the instructions in storage930 can implement a PIT module 932, a FIB module 934, and a contentstore manager 936, all of which can be in communication with each otherthrough various means.

In some embodiments, the Content Store can be a compressed table thatincludes an index to the storage array's network address, a flag toindicate the type of encapsulation to construct, and the disk addressesto retrieve. The NPU constructs the appropriate network frame or frameswhen needed. This approach trades storage space in the CS for NPUprocessing cycles.

In some embodiments, modules 932, 934, and 936 can be partially orentirely implemented in hardware and can be part of processor 910.Further, in some embodiments, the system may not include a separateprocessor and memory. Instead, in addition to performing their specifictasks, modules 932, 934, and 936, either separately or in concert, maybe part of general- or special-purpose computation engines.

Storage 930 stores programs to be executed by processor 910.Specifically, storage 930 stores a program that implements a system(application) for direct access to an attached storage, such as anetwork-attached storage 940. During operation, the application programcan be loaded from storage 930 into memory 920 and executed by processor910. As a result, system 900 can perform the functions described above.System 900 can be coupled to an optional display 980, keyboard 960, andpointing device 970, and also be coupled via one or more networkinterfaces to network 982.

The data structures and code described in this detailed description aretypically stored on a computer-readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. The computer-readable storage medium includes, but is notlimited to, volatile memory, non-volatile memory, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs),DVDs (digital versatile discs or digital video discs), or other mediacapable of storing computer-readable media now known or later developed.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in acomputer-readable storage medium as described above. When a computersystem reads and executes the code and/or data stored on thecomputer-readable storage medium, the computer system performs themethods and processes embodied as data structures and code and storedwithin the computer-readable storage medium.

Furthermore, methods and processes described herein can be included inhardware modules or apparatus. These modules or apparatus may include,but are not limited to, an application-specific integrated circuit(ASIC) chip, a field-programmable gate array (FPGA), a dedicated orshared processor that executes a particular software module or a pieceof code at a particular time, and/or other programmable-logic devicesnow known or later developed. When the hardware modules or apparatus areactivated, they perform the methods and processes included within them.

The above description is presented to enable any person skilled in theart to make and use the embodiments, and is provided in the context of aparticular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

What is claimed is:
 1. A computer-executable method for caching contentdata to a storage device attached to a node in a content-centric network(CCN), the method comprising: receiving, by the node, a content packet;forwarding the content packet to an incoming port of an interest in thecontent packet; caching a copy of the content packet in the attachedstorage device; assembling a query, which includes at least a networkheader and an address associated with storage blocks at which the cachedcopy of the content packet is located, corresponding to the contentpacket; and storing the query in a cache table, thereby facilitating asubsequent direct access to the storage blocks using the assembled queryto retrieve the copy of the content packet.
 2. The method of claim 1,wherein the attached storage device includes at least one of: anadvanced technology attachment (ATA) over Ethernet storage array; aFibre Channel over Ethernet (FCoE) storage array; and an Internet SmallComputer System Interface (iSCSI) storage array.
 3. The method of claim2, wherein the network header includes an Ethernet header.
 4. The methodof claim 1, wherein the content packet includes a plurality offragments, wherein caching a copy of the content packet involves storingeach fragment into a separate set of contiguous storage blocks, whereina separate query is assembled for each fragment, and wherein an entry inthe cache table corresponding to the content packet includes a pluralityof queries corresponding to the plurality of fragments.
 5. The method ofclaim 1, further comprising: subsequently receiving an interest in thecontent packet; searching the cache table to obtain the query; sendingthe query to the attached storage device; receiving, from the attachedstorage device, a data packet in response to the query; removing anetwork header from the received data packet; and forwarding the datapacket.
 6. The method of claim 5, wherein searching the cache tableinvolves performing a longest-prefix-match lookup based on the receivedinterest.
 7. The method of claim 5, further comprising constructing anetwork representation of the query based on elements stored in thecache table.
 8. The method of claim 1, wherein the address associatedwith the storage blocks includes a logical block addressing (LBA)address.
 9. The method of claim 1, wherein the node includes multipleline cards each capable of receiving content and interest packets, andwherein the cache table is populated to all of the multiple line cards.10. An system for caching content data to a storage device attached to anode in a content-centric network (CCN), the system comprising: aprocessor; a second storage device coupled to the processor and storinginstructions which when executed by the processor cause the processor toperform a method, the method comprising: receiving a content packet;forwarding the content packet to an incoming port of an interest in thecontent packet; caching a copy of the content packet in the attachedstorage device; assembling a query, which includes at least a networkheader and an address associated with storage blocks at which the cachedcopy of the content packet is located, corresponding to the contentpacket; and storing the query in a cache table, thereby facilitating asubsequent direct access to the storage blocks using the assembled queryto retrieve the copy of the content packet.
 11. The system of claim 10,wherein the attached storage device includes at least one of: anadvanced technology attachment (ATA) over Ethernet storage array; aFibre Channel over Ethernet (FCoE) storage array; and an Internet SmallComputer System Interface (iSCSI) storage array.
 12. The system of claim11, wherein the network header includes an Ethernet header.
 13. Thesystem of claim 10, wherein the content packet includes a plurality offragments, wherein caching a copy of the content packet involves storingeach fragment into a separate set of contiguous storage blocks, whereina separate query is assembled for each fragment, and wherein an entry inthe cache table corresponding to the content packet includes a pluralityof queries corresponding to the plurality of fragments.
 14. The systemof claim 10, wherein the method further comprises: subsequentlyreceiving an interest in the content packet; searching the cache tableto obtain the query; sending the query to the attached storage device;receiving, from the attached storage device, a data packet in responseto the query; removing a network header from the received data packet;and forwarding the data packet.
 15. The system of claim 14, whereinsearching the cache table involves performing a longest-prefix-matchlookup based on the received interest.
 16. The system of claim 14,wherein the method further comprises constructing a networkrepresentation of the query based on elements stored in the cache table.17. The system of claim 10, wherein the address associated with thestorage blocks includes a logical block addressing (LBA) address. 18.The system of claim 10, wherein the node includes multiple line cardseach capable of receiving content and interest packets, and whereinmethod further comprises populating the cache table to all of themultiple line cards.
 19. A non-transitory computer-readable storagemedium storing instructions that when executed by a computer cause thecomputer to perform a method for caching content data to a storagedevice attached to a node in a content-centric network (CCN), the methodcomprising: receiving a content packet; forwarding the content packet toan incoming port of an interest in the content packet; caching a copy ofthe content packet in the attached storage device; assembling a query,which includes at least a network header and an address associated withstorage blocks at which the cached copy of the content packet islocated, corresponding to the content packet; and storing the query in acache table, thereby facilitating a subsequent direct access to thestorage blocks using the assembled query to retrieve the copy of thecontent packet.
 20. The computer-readable storage medium of claim 19,wherein the attached storage device includes at least one of: anadvanced technology attachment (ATA) over Ethernet storage array; aFibre Channel over Ethernet (FCoE) storage array; and an Internet SmallComputer System Interface (iSCSI) storage array.
 21. Thecomputer-readable storage medium of claim 20, wherein the network headerincludes an Ethernet header.
 22. The computer-readable storage medium ofclaim 19, wherein the content packet includes a plurality of fragments,wherein caching a copy of the content packet involves storing eachfragment into a separate set of contiguous storage blocks, wherein aseparate query is assembled for each fragment, and wherein an entry inthe cache table corresponding to the content packet includes a pluralityof queries corresponding to the plurality of fragments.
 23. Thecomputer-readable storage medium of claim 19, wherein the method furthercomprises: subsequently receiving an interest in the content packet;searching the cache table to obtain the query; sending the query to theattached storage device; receiving, from the attached storage device, adata packet in response to the query; removing a network header from thereceived data packet; and forwarding the data packet.
 24. Thecomputer-readable storage medium of claim 23, wherein searching thecache table involves performing a longest-prefix-match lookup based onthe received interest.
 25. The computer-readable storage medium of claim23, wherein the method further comprises constructing a networkrepresentation of the query based on elements stored in the cache table.26. The computer-readable storage medium of claim 19, wherein theaddress associated with the storage blocks includes a logical blockaddressing (LBA) address.
 27. The computer-readable storage medium ofclaim 19, wherein the node includes multiple line cards each capable ofreceiving content and interest packets, and wherein the cache table ispopulated to all of the multiple line cards.